The present invention relates to testing integrated circuits (IC's), and more specifically, to testing for normal or reverse temperature related delay variations in integrated circuits.
Over the past few technology generations, increased power densities and low-power techniques have made the effects of temperature variation on integrated circuits (or “chips”) an increasingly important problem. Temperature changes may affect, for example, signal propagation delays in the IC. Thus, awareness of temperature related performance effects is critically important for maintaining system functionality.
Increasing power densities affect temperature in at least two ways. For a single circuit on an IC, higher densities mean a larger temperature difference between when the circuit is idling and when it is switching heavily. This increase in temperature differences also directly affects system delays. For an IC as a whole, the higher power densities result in a larger gradient between a high-activity core and low-activity nest logic, approaching 50° C. in some cases.
The delay dependence is proportional to the operating voltage. This means that low-power techniques such as dynamic voltage scaling make it even more difficult to determine the actual circuit latency. In newer technologies, this problem is compounded by the reverse temperature effect (which occurs at higher voltages as technologies scale). The “reverse” domain (or reverse temperature dependence domain) is the voltage domain in which an increase in temperature causes an increase in on-current and increases circuit speed. This is opposed to general (“normal”) circuit behavior where an increase in temperature reduces the on-current and reduces circuit speed. This occurs below some voltage threshold, usually called the temperature-invariant voltage. In both the normal and reverse domains the leakage current still has the same exponential dependence on temperature. The reason for this reversal at lower voltages is a function of the changing proximity to the threshold voltage—the temperature effects on mobility and threshold voltage trade-off, meaning that as temperatures increase, delay can increase or decrease depending on the operating voltage.